Add integration with the 'rmarkdown' package

This change adds hooks on the UI and server to support the rmarkdown package. In particular:

- The **Knit** command now uses rmarkdown to render the document, if the document is an R Markdown file and the rmarkdown package is installed.
- New code on the server runs the render in a separate R process, and a new console tab in the client shows the output of the process as it runs. 
- A new satellite window shows the render output (limited to HTML for now, with PDF as supported inline by the browser), modified to use the rmarkdown-supplied version of the MathJax library if necessary.